1. Field of the Invention
The present invention relates in general to a vehicle steering system, and more specifically to a controlled torque steering system for an on demand hydraulic assist steering system.
2. Description of the Related Art
There is a wide variety of steering systems employed on vehicles. The first steering systems used on vehicles were the manual steering systems. Some of these type systems employed a Gemmer worm-and-roller gear, and/or a recirculating ball gear. Manual steering systems have been used on many automobiles well into the 1970's.
Conventional rack and pinion power steering systems (non-variable assist) are still widely used today. These are primarily hydraulic systems that may include a recirculating ball gear with a rack and pinion.
More recently, variable assist power steering systems have been developed and implemented as early as 1988. This design is an improvement over conventional power steering systems and provides the advantages of better road feel and increased fuel economy. The variable assist power steering system uses an electronic variable orifice (EVO) actuator or system, a controller, and a hand or steering wheel speed sensor. Vehicle speed is inputted into the EVO system controller so that as vehicle speed changes so does the steering assist to provide a more controlled driving experience.
It is known that an on demand hydraulic steering system uses less horsepower than conventional power steering.
These recent power steering systems have included speed sensitive steering which increase the manual steering forces or the driver feel at high vehicular speeds by reducing hydraulic fluid flow through control valves in order to reduce valve gain. Examples of such variable assist power steering systems are described in U.S. Pat. No. 5,133,424 entitled "Variable Assist Power Steering Control Valve" which issued on Jul. 28, 1992, and U.S. Pat. No. 4,877,099 entitled "Electronically Controlled Variable Assist Power Steering System" which issued on Oct. 31, 1989, of which both patents are incorporated by reference herein.
In an on demand hydraulic assist steering system, typically the system includes a hydraulic power cylinder coupled to the linkage mechanism, and a pump or source of pressurized hydraulic fluid. A valving mechanism directs the hydraulic fluid from the pump to the power cylinder in relation to the magnitude and direction of steering torque applied to the steering wheel. In this manner, there is hydraulically generated a steering assist force on the linkage mechanism which aids the operator of the vehicle.
At low vehicle speeds, a relatively high level of steering assist is likely to be required and the hydraulic flow is maintained at a relatively high level under such conditions to provide this capability. At high vehicle speeds, a relatively low level of steering assist is likely to be required and the hydraulic flow is reduced. Potentially, this can effect an energy savings and can improve the on-center stability of the vehicle. U.S. Pat. No. 5,725,023 describes an electronically variable pressure power steering system with control valve and sensor. This patent is hereby incorporated by reference.
Other variable assist power steering systems are described in the following patents: U.S. Pat. No. 5,307,892 entitled "Electronically Controlled Power Steering System"; U.S. Pat. No. 5,207,780 entitled "Hydraulic Valve Device for Automotive Power Steering Gear"; U.S. Pat. No. 4,828,065 entitled "Electronically Controlled Power Steering System"; U.S. Pat. No. 4,471,811 entitled "Linear Solenoid-Operated Valve for Use in Power Steering System"; U.S. Pat. No. 4,643,227 entitled "Linear Solenoid-Operated Valve for Use in Power Steering System"; and U.S. Pat. No. 4,629,025 entitled "Flow Regulated Power Assist Steering System with Evasive Maneuver Capability". Other patents related to this disclosure include: U.S. Pat. No. 5,513,832 entitled "Variable Force Solenoid Valve"; U.S. Pat. No. 5,513,673 entitled "Electrically Modulated Pressure Regulator Valve with Variable Force Solenoid"; U.S. Pat. No. 5,261,637 entitled "Electrical Variable Orifice Actuator"; and U.S. Pat. No. 4,947,893 entitled "Variable Force Solenoid Pressure Regulator for Electronic Transmission Controller". These patents are all hereby incorporated herein by reference.
When a torque sensor located in the steering wheel shaft outputs a torque signal, the electronic controller ordinarily outputs a command signal to the electronically variable orifice system (EVO) which results in more flow to the steering gear. However, when the torque demand decreases or is less, an oscillation can occur which is caused by unstable electronics and/or hydraulics.
Thus, there still exists a need for a system that addresses the problems with such oscillations to provide a more stable, better controlled steering system. Preferably, such a system would delay or ramp down the output signal to the electronically variable orifice or to a slower rate. Advantageously, such a system would include a selecting means for operator or user interface that would include a user actuatable switch with means for selecting a desired amount of assist when activated. In the inactive state, the selecting means would automatically default to the manufacturer's variable assist power steering system.